I like special features too, and I looked a lot into what's on the new set and what isn't.
This page tells you a lot about what's on there. Scroll down to "The Supplements".
Basically everything on that fourth disk in your original set is on the blu-ray set. When the movies were released individually in 2008, they added more features on the new disks as well as new introductions with Spielberg and Lucas. Some of that content is included in the blu-ray set, but not all. For example, the introductions are not there.
There are two nice documentaries on the first disk (DVD) of KOTCS, only one of which is on the blu-ray set. Most of KOTCS's second disk of features has been cut down or left out entirely on the blu-ray set.
There are two new documentaries on the blu-ray set, which (if I recall) are more like on-the-set during production, following the cast and crew. No interviews, but it's nice watching like you're there, and it includes some peaks at deleted scenes.
I hate the Spielberg/Lucas approach to releases: holding back some features, making some available only on sets or individual releases. I find it greedy and manipulative. Same thing with the various Star Wars releases. I don't feel like rewarding them with buying incomplete things repeatedly - and I'm sure they're not done. There
will be UHD sets, there
will be all inclusive sets, there
will be Star Wars theatrical HD sets, and there
will be Spielberg commentaries no matter what he says now. As long as people pay. They also keep retailers from dropping prices and punish those who sell used. (Disney is far worse at that.)
What I did was buy the 2008 individual releases, the two-disk DVD of KOTCS, and found the very nicely done 4th DVD disk of features individually on ebay. (I didn't get a blu-ray player until 2011). So I have all non-blu-ray features. Then I checked out the bluray set from the public library to watch the new features. I will get the blu-ray set used when it gets inexpensive. Check ebay, amazon, or your local FYE store for used.
Hope that helps.